In the past twenty or so years, hand-held string trimmers using flexible and expendable filament have become very common. Many patents have been issued for devices that store and feed out filament, mainly the familiar bump feed heads, plus heads that automatically feed out cord with very limited operator influence.
Among the heads that automatically feed out filament almost independently of operator control, research has identified these:
______________________________________ 4,561,180 (1985) to Pittinger Bump feed to incrementally feed the flail 4,236,312 (1980) to Lombard and Electric motor maintains Foster constant flail length 4,285,127 (1981) to Zerrer Speed actuates cams 4,290,200 (1981) to Lombard Speed actuates a cam 4,347,666 (1982) to Moore Speed actuates a cam 4,366,621 (1983) to Mitchell Speed actuates a cam 4,366,622 (1983) to Lombard Speed actuates a cam 4,483,069 (1984) to Moore Speed actuates a cam 4,835,867 (1989) to Foster, Speed actuates a cam Collings and Stuart 4,852,258 (1989) to Foster Speed actuates a cam ______________________________________
Trimmer heads using the principles described in these patents have moving parts which collect grit and grime and very often fail to function acceptably or to function at all. These heads also have spools on which the cutting filament wraps over on top of itself, then refuses to feed, a very common problem with the bump feed heads too.
The market for heads with manually-installed flails will remain good until a trimmer head that feeds dependably--with few or no malfunctions--comes on the market.